The growth of the mammalian lens must be regulated with precision if images are to be focused sharply on the retina. Lens growth is ultimately dependent on cell proliferation in the epithelial layer but, despite many years of study, the details of this process are poorly understood. It has proved difficult to measure cell dynamics on the spherical anterior surface of the lens. New approaches, utilizing induced expression of fluorescent proteins and in situ imaging of S-phase cells, have provided fresh insights into the kinetics of lens cell proliferation. Preliminary data suggest that most epithelial cells are locatd in regions of the epithelium that support cell proliferation. Long-term imaging studies have revealed that cells migrate within the epithelium and, unexpectedly, that epithelial cells undergo multiple divisions before terminally differentiating into fiber cells. As a consequence, large, cuneiform (wedge- shaped) clones of fiber cells in the adult lens are derived from individual epithelial cells. Intriguingly, cortical cataracts in humans are characterized by the presence of cuneiform opacities of unknown etiology. Epidemiological studies have identified a link between cortical cataracts and sun exposure and some investigators have suggested that UV-induced somatic mutations could contribute to cataract formation. New insights into proliferative dynamics in the lens epithelium provide an explanation for how mutations incurred in sun-exposed regions of the epithelium could manifest as cuneiform opacities in the lens fiber mass. Kinetic parameters will be collected and used to formulate a stochastic, branching process model of lens growth. A cellular model will also be developed, in which induced expression of GFP will be used as a lineage tracer to follow the fate of mutant cells. Finally, mutations in TP53, a gene commonly mutated in sun-exposed epidermis, will be used to monitor mutational load in lens epithelium and fiber cells. These experiments are expected to provide important insights into the control of lens growth and the etiology of cortical cataract in humans.